1. Field of the Invention
The present invention relates to the thermal modeling of a switched reluctance motor, and particularly a switched reluctance motor used for supercharging an internal combustion engine.
2. Description of Related Art
The electric currents and friction in an electric motor generate heat that can damage motor components such as electrical connections, electrical insulation or motor bearings. These problems are particularly acute in an electric motor used to power a supercharger coupled to an engine owing to the need for high rotational speeds, for example 70,000 rpm, needed to spin a supercharger turbine. Such a motor can consume 300 A of current at 12 V during continuous operation. When the supercharger is initiated, transient currents as high as 450 A may be needed in order to reach an operational rotational speed within about 1 s. It is also necessary to maintain close mechanical tolerances in the bearings and between the rotor and stator. It is therefore necessary to maintain the temperature of the motor components below allowable limits in order to prevent damage to the motor and to achieve a useful operational lifetime.
The temperature of some parts of the motor, for example the stator or bearings, may be measured with a temperature sensor. This, however, adds cost and complexity, as it then becomes necessary to position the sensors within or close to the component for which a temperature reading is needed. The sensors must then also be wired to the supercharger controller. For some components, particularly a rotor, it may not be possible or economic to provide a temperature sensor.
One solution to this problem may be to calculate the temperature of the motor components, using a mathematical model. A very accurate calculation could be performed using a finite element model, taking as its inputs known supercharger operational parameters. To be useful, however, a model must be capable of generating results in near-real-time, for example, within 0.1 S. An accurate finite element model to calculate supercharger temperatures will be too slow to run using a typical automotive electronic processor.
It is an object of the present invention to provide an apparatus and method for modeling the temperatures in an electric motor for supercharging an internal combustion engine.
According to the invention, there is provided a supercharger system for an internal combustion engine, comprising:
a supercharger adapted to supply air to the engine, the supercharger being driven by an electric motor having a rotor, the rotor rotating at an angular speed xcfx89 when the supercharger is energized with electric current and the supercharger thereby drawing a mass airflow volume V;
a controller adapted to operate the supercharger;
an angular speed sensor coupled to the controller a measure the angular speed xcfx89 of the rotor;
an air flow volume sensor coupled to the controller to measure the mass airflow volume V;
wherein the controller is adapted to calculate the temperature T of at least one component of this supercharger, using the measures of the rotor angular speed xcfx89 and the mass airflow volume M, wherein the equation to calculate T is:
T(tn)=aT(tnxe2x88x921)+bxcfx89(tnxe2x88x921)+cV(tnxe2x88x921)+dxcfx89(tnxe2x88x921)T(tnxe2x88x921)+eV(tnxe2x88x921)T(tnxe2x88x921)xe2x80x83xe2x80x83[1]
where:
the calculation is performed iteratively at time intervals at xcex94t starting at an initial time t0 and for subsequent times tn=tnxe2x88x921+xcex94t, n=1,2,3 . . . ;
T(tn) is the calculated temperature of the component at a time tn;
T(tnxe2x88x921) is the calculated temperature of the component at a time tnxe2x88x921;
V(tnxe2x88x921) is the measured mass airflow volume at time tnxe2x88x921;
xcfx89(tnxe2x88x921) is the rotor angular speed at time tnxe2x88x921;
T(t0) is a known temperature of the component at the initial time t0; and
a, b, c, d, and e are constant values.
Also according to the invention, there is provided a method of controlling the operation of a supercharger system for an internal combustion engine, the system comprising a supercharger (10) with a rotor (31) for supplying air to the engine, an electric motor (14) for driving the supercharger (10), and a controller (32) adapted to operate the supercharger (10), comprising the steps of:
using the electric motor (14) to drive the supercharger (10) at an angular speed xcfx89 and to draw through the supercharger (10) a mass airflow volume V;
measuring the angular speed xcfx89 the rotor;
measuring the mass airflow volume V of the supercharger;
calculating the temperature T of at least one component of the supercharger, using the measures of the rotor (31) angular speed xcfx89 and the mass airflow volume V;
wherein the equation to calculate T is:
T(tn)=aT(tnxe2x88x921)+bxcfx89(tnxe2x88x921)+cV(tnxe2x88x921)+dxcfx89(tnxe2x88x921)T(tnxe2x88x921)+eV(tnxe2x88x921)T(tnxe2x88x921)xe2x80x83xe2x80x83[1]
where:
the calculation is performed iteratively at time intervals xcex94t starting at an initial time t0 and for subsequent times tn=nxe2x88x921+xcex94t, n=1,2,3 . . . ;
T(tn) is the calculated temperature of the component at a time tn;
T(tnxe2x88x921) is the calculated temperature of the component at a time tnxe2x88x921;
V(tnxe2x88x921) is the measured mass airflow volume at time tnxe2x88x921;
xcfx89(tnxe2x88x921) is the rotor angular speed at time tnxe2x88x921;
T(t0) is a known temperature of the component at the initial time t0; and
a, b, c, d, and e are constant values
controlling the operation of the supercharger (10) in accordance with said calculated temperature.
A main advantage of the invention is that it makes use of temperature measurements that are normally available in an automotive environment, for example ambient temperature, the temperature of engine coolant, or the temperature of the inlet air. From this, it is possible to calculate the initial temperature of the various components of the supercharger.
The form of bilinear equation [1] can readily be implemented in software in existing engine control unit hardware to achieve a near real-time calculation of supercharger motor temperatures, and so imposes no additional cost burden in terms of improved computational electronics.
The controller may then be adapted to calculate the temperature TR(tn) of the rotor.
If the rotor spins on bearings, the controller may be adapted to calculate the temperature TRB(tn) of the rotor bearings according to equation [1].
The electric motor will generally have a stator and the controller may then be adapted to calculate the temperature TS(tn) of the stator according to equation [1].
The temperature of stator windings TSW(tn) through which electrical current flows when the electric motor is energized may also be calculated according to equation [1].
In general, the stator will have a stator core through which magnetic flux is concentrated when the electric motor is energized. The controller may then be adapted to calculate the temperature TSC(tn) of the stator core according to the equation
TSC(tn)=ATSC(tnxe2x88x921)+BTSW(tnxe2x88x921)+CTSW(tnxe2x88x921)TSC(tnxe2x88x921)xe2x80x83xe2x80x83[2]
where
the calculation is performed iteratively at time intervals xcex94t starting at an initial time t0 and for subsequent times tn=tnxe2x88x921+xcex94t, n=1,2,3 . . .
TSC(tn) is the calculated temperature of the stator core component at a time tn;
T(tnxe2x88x921) is the calculated temperature of the stator core at a time tnxe2x88x921;
TSC(t0) is a known temperature of the stator core at the initial time t0;
TSW(tnxe2x88x921) is the temperature of the stator windings at a time tnxe2x88x921, calculated according to equation [1]; and
A, B and C are constant values.
This equation makes us of previously calculated values for the temperature TSW(tnxe2x88x921) of the stator windings, and so places minimal additional computational requirements on electronic hardware.
The engine may comprise one or more sensors for providing the controller a measure of the temperature of one or more engine operating parameters. The controller may then be adapted to calculate said initial temperature T(t0) from said measure(s) of one or more engine operating parameters.